QUANTITATIVE MICROTEXTURAL STUDY OF THE SILT FRACTION IN MUDROCKS

To understand the history and physical properties of mudrocks requires data from the silt (2-63 μm) grain-size fraction. The volumetic content of silt grains in mudrocks reaches 40% or more and may partly control porosity. Mudrock microtextural variations between basins with contrasting histories may help to differentiate the relative importance of transport, deposition, and diagenesis in mudrock evolution. We have manually digitized silt grains using scanning electron microscope (SEM) images with elemental X-ray maps, and quantified the mudrock microtexture using Jmicrovision© software. We applied this approach to samples from: the Cretaceous Pearsall Formation of the Maverick Basin, the Mississippian Barnett Shale of the Fort Worth Basin, and the Devonian Marcellus Formation of the Appalachian Basin. All three formations are known hydrocarbon source rocks from predominantly siliciclastic clastic systems that have been buried to depths that cover a range of diagenetic conditions. However, each sample set provides specific opportunities: (i) the Pearsall allows us to track changes from the up-dip to the down-dip section of the regional Gulf of Mexico strata, (ii) the Barnett allows us to contrast microtexture across differing porosities, and (iii) the Marcellus allows us to see how these mudrocks respond to substantial increases in temperatures. In the Pearsall and Barnett formations, the preferred orientation of slightly elongate fine silt grains creates an anisotropy that dominates the fabric. In this 2D slice there are fewer observed grain contacts and more clay-size matrix. In contrast, preliminary results from the Marcellus appear to show more contacts between coarser silt particles, with only interstitial clay-size matrix. These microtextural differences may contain important clues to mechanical versus chemical diagenetic histories of these units. A key result from the study is that the relative abundance of detrital carbonate and silicate, and the presence of authigenic minerals has little effect on grain-size distribution. The silt framework and primary grain size distribution are preserved despite the diagenetic histories experienced by the mudrocks within these basins.